Echo cancellation using cross-correlation of buffered receive and transmit sample segments to determine cancelling filter coefficients

ABSTRACT

An echo detecting system includes data stores for storing signals from up and down channels respectively. The signals undergo pre-processing to identify signals forms characteristic of speech and instruct a measurement unit to carry out comparison using cross-correlation techniques between the signals stored in the stores only when such characteristics are detected. This reduces the processing power required and raises the accuracy of the correlations. Parallel processing techniques allow echoes with longer delay periods to be detected. The results of the measurement may be used to generate an echo-cancellation signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the measurement of signal quality overtelecommunications links, and in particular to the detection ofinterference.

2. Related Art

More particularly, this invention relates to the detection of spurioussignals generated on a second channel as a result of signals beingtransmitted on a first channel, a situation known generally as`crosstalk`. The spurious signal, once detected, may be measured andcancelled.

The invention is particularly suited to detecting echo. This occurs in atwo-way telecommunications link. A signal travelling in a firstdirection gives rise to a spurious signal travelling in the oppositedirection. If this spurious signal returns to the original source of thesignal, it will appear as an echo.

The echo effect may be caused in one of several ways. It may occur as aresult of acoustical feedback between the earpiece and mouthpiece of atelephone. It may occur as a result of reflections caused by impedancemismatches. It may occur as a result of cross-coupling between the pathsin 4-to-2 wire hybrid points; these are the points where the two-waytraffic carried over the two-wire connection from a telephonetermination is separated into two separate channels (a so-calledfour-wire connection). This invention is suitable for detecting echoeffects at points in the system where signals in the two directions arecarried over two separate channels.

The result of any such echo effect is that a speaker will receive his orher own speech, delayed by a short period. The magnitude of the delay islargely determined by the distance the signal has to travel, with asmaller contribution from signal processing delays. The distancestravelled by signals in intercontinental calls can introduce delaysreadily detectable by human observers: the round trip distance over theearth's surface between one point on the earth's surface and itsantipodes is 40000 km (approximately 140 light-milliseconds--sincelandlines do not necessarily follow the shortest route the practicaldistance is greater than this). The round trip distance between twopoints on the earth's surface via a geostationary satellite isapproximately 1/2 light-second (150000 km). International call-diversionand other network services can create even longer paths.

Delays of this order of magnitude, as well as being annoying, alsoconfuse the speaker who can find it impossible to continue speaking. Itis therefore desirable to detect when echo is occurring so that remedialaction can be taken. This remedial action may involve taking the faultycircuit out of use until it can be repaired, or limiting the use of thefaulty circuit to uses where the echo causes less problems, such asshort-distance calls (in which the echo delay is too short to betroublesome) or to one-way transmission such as facsimile transmissions.Methods also exist for cancelling the echo signal by combining itartificially with a complementary signal derived from the outgoingsignal to generate a zero output. However, all these systems requireprior knowledge that an echo exists, and something of itscharacteristics, notably Its delay time and its attenuation.

It is known to transmit test signals over a telecommunications link inorder to detect the presence of echoes. This system can only be used onlines which are not currently in use, because traffic on the line wouldinterfere with the detection of the test signal echo. It is also Knownto use trained human observers to monitor live conversations, but thismethod is labour-intensive, subject to human subjectivity, and also hasimplications for the privacy of the speakers.

In-service non-intrusive measurement systems are known which use leastmean square (LMS) adaptive filter systems to measure the delay and echostrength from the conveyed impulse response. Modern digital signalprocessors can support about 650 filter coefficients: at a sampling rateof 8 kHz this equates to a maximum detectable echo path of about 80 ms.To detect longer echo paths using this impulse response technique eitherthe number of filter coefficients must be increased beyond thesepractical limits, or the sampling rate must be reduced, which reducesthe likelihood of a convergent response from any one sample.

U.S. Pat. No. 5,062,102 (Taguchi) discloses an echo canceller in whichecho is detected by identifying cross-correlations between the signalscarried by first and second transmission lines using a cross-correlationtechnique. This allows short samples of signals To be used, rather thanthe long passages required by an adaptive filter, and allows filtercoefficients for echo correction signals to be generated more rapidlythan by the use of adaptive filters.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, an interference detectionsystem for a telecommunications link having separate first and secondchannels, the system comprising first monitoring means for monitoringsignals travelling over the first channel, second monitoring means formonitoring signals travelling over the second channel and comprisingcomparison means for comparing the signals monitored by the first andsecond monitoring means for one or more delay periods to identify thepresence of interference between the channels, wherein the comparisonmeans is arranged to identify cross-correlations between the signalsmonitored by the first and second monitoring means, is characterised inthat the first monitoring means includes means for detecting andselecting signal segments on the first channel having predeterminedcharacteristics and the comparison means is arranged to identifycross-correlations between such characteristic signal segments and thesignals monitored by the second monitoring means, the selected signalsegments having lengths corresponding to the duration of thepredetermined characteristic.

According to a second aspect, a method of detecting interference betweenchannels on a telecommunications link having first and second channels,the method comprising the steps of monitoring signals travelling over afirst channel, monitoring signals travelling over a second channel, andcomparing the signals for one or more delay periods to identify thepresence of interference between the channels, wherein the methodcomprises the identification of cross-correlations between the signalscarried by the first and second channels, is characterised by thefurther steps of detecting signals having predetermined characteristicson the first channel, selecting segments of signals having saidcharacteristics, and identifying cross-correlations between suchcharacteristic signal segments and the signals carried by the secondchannel, the selected signal segments having lengths corresponding tothe duration of the predetermined characteristic.

By selecting such characteristic signals for analysis the processingcapacity available can be used efficiently by concentrating an signalsamples which are likely to produce strong cross-correlations, allowinga wider range of delays to be monitored for. False correlations fromlow-level white noise are also avoided.

In a preferred arrangement signal segments having such characteristicsare detected on the first channel and selected for attempted correlationwith the signals carried by the second channel, the selected signalsegments having lengths corresponding to the duration of thepredetermined characteristic, and preferably greater than apredetermined minimum. By tailoring the sample length to match theduration of the characteristic element, the chance of a correctcorrelation being made is improved, because the longer the sample theless likely a false correlation is made, without wastefully attemptingto correlate parts of a signal not containing the characteristicelements.

The signal characteristics to be identified may include signal strengthor may be characteristics associated with human speech. Becausecharacteristic features of the signals are monitored and correlated,these features can also be used to determine other characteristics ofthe interference phenomenon.

In a preferred arrangement, the comparison means comprises a pluralityof cross-correlation means, each cross-correlation means performing across-correlation for a different delay period, and delay measurementmeans for determining, from the outputs of the cross-correlation means,the magnitude of the delay in the interference signal.

The invention can be used for monitoring interference between any twochannels of a communications system, but is particularly suited to echodetection provided that the send and receive paths are separated, e.g.conventional four-wire analogue telephony, digital telephony, broadbandapplications, duplex radio systems (time division or frequency division)or asynchronous transfer mode (ATM). Accordingly, the pair or pairs ofchannels of the communications system to which the intereferencedetection system is connected preferably each comprise a two-waycommunications link, the system being arranged to detect echo.

Embodiments of the invention allow a greater range of delay periods tobe monitored simultaneously by storing several samples in separatestores and processing each separately. In a typical situation twodifferent echo delay periods will be found, depending on which caller isspeaking.

The system nay be used to provide input to an echo canceller. An echocanceller adds to the return path a cancellation signal corresponding tothe signal on the outward path, having a delay and attenuationcorresponding to that of the echo, but having opposite phase. Oneproblem encountered with known echo cancellers is that a falsecorrelation can cause a cancellation signal to be inserted where none isneeded, which creates its own echo effect. The problem can be avoided bydetermining a rolling average from a predetermined number ofmeasurements from the delay and/or attenuation measurement means,differing from each other by values less than a predetermined value. Theeffects of individual false correlations, which will have differentattenuations and delays from the true echo, are therefore minimised.

In a network management system there may be a plurality of interferencedetection systems, each associated with a respective pair of channels,and one or more means for introducing a cancellation signal into achannel on which interference is detected. By arranging the system inthis way the number of cancellers can be reduced, the cancellers beingdynamically allocated to those channel pairs where interference, or themost serious interference, is detected.

The system may include a speech direction determination means comprisingmeans for determining on which channel the longest segments of signalhaving the monitored characteristics occur. The characteristic featuresof the incoming signal can therefore be used to identify which of thetwo callers is speaking, and therefore which path should be monitoredfor echo signals, thus reducing the processing overhead by a furtherfactor of two.

The length of the delay can be used to assist in locating the source ofthe echo, as longer delays are caused by equipment further away, or withmore intermediate processing elements. Insofar as the call routing isknown, a network operator can thereby identify the faulty apparatus. Ofcourse, in some cases the call may be an interconnection between twooperators, and one operator may not know the routing in the otheroperator's network. In this case, a network operator using the echodetection system of the invention can nevertheless identify from thelength of the echo delay whether the echo is caused by his own networkor the other one, and thus whether remedial action is within his power.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of exampleonly, with reference to the drawings, in which

FIG. 1 illustrates a simple telephone network including an echo detectoraccording to the invention.

FIG. 2 shows the various elements of one embodiment of the echo detectorof FIG. 1, incorporating an echo canceller.

FIG. 3 shows a echo loss measurement system incorporating an echodetector according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a simplified telephone network having two terminations 1and 2 connected through respective 4-to-2 wire hybrids 3, 4 to atelephone trunk link having a first path 5, (from hybrid 3 to hybrid 4)and a second path 15 in the reverse direction. Connected at some pointalong the paths 5 and 15 is a non-intrusive measuring device 6 which isdescribed in more detail below. The device 6 is connected to the firstpath 5 at point X and to the second path 15 at point Y. FIG. 2 shows theecho-detector of FIG. 1 in more detail. From the tap points X, Y on thepaths 5, 15 respectively signals are fed to respective buffers 8, 18,and hence to respective pre-processing units 9, 19. The pre-processingunits 9, 19 feed a speech direction classification unit 11. Ameasurement unit 10 receives inputs from the data buffers 8, 18,pre-processing units 9, 19, and direction classification unit 11, andsupplies an output to a post-processing unit 12 which in turn providesan output to one or other of two echo cancelling units 7, 17, which alsoreceive an input from respective data buffers 8, 18. The echo cancellers7, 17 provide an input to the paths 15, 5 respectively throughrespective combiners 13, 14 downstream of the tap points X, Y.

FIG. 3 illustrates an echo loss calculation device which can make use ofthe output of the device according to the invention.

Two signals X, Y are input to a speech classifier 11 which, as in FIG.2, identifies which signal is the incident signal and which thereflected signal, and controls switches 36, 37 to feed the incidentsignal to an input 21 and the reflected signal to an input 30.

The incident and reflected signals are fed via buffers 8, 18 to aprocessor 10, as in FIG. 2, and the output of the processor 10 is fed toa bulk delay buffer 22.

The incident signal at input 21 is input to the buffer 22 to delay itfor a period corresponding to the echo delay determined by thepost-processor 12, generating a delayed input 23. Both signals are thenfed to respective modifiers 26, 27 in which weightings are applied togenerate a modified delayed input signal 28 and a modified reflectedsignal 31. The weightings are derived from an analysis unit 24monitoring the delayed incident signal 23. The modified input signal 28is then fed to a digital analogue filter (DAF) 29. The output 32 of thefilter 29 is compared with the modified reflected signal 30 in acomparator 35 to generate an error signal 33 which is fed back to theDAF 29. The filter values of the DAF 29 can be read off at an output 34to allow calculation of the echo loss by a calculator 38.

The operation of the invention will now be described. Referring now toFIG. 1, echo can be caused when part of a signal travelling along firstpath 5, destined for termination 2, is reflected at hybrid 4 andreturned over second path 15. This signal will be heard by the user oftermination 1, who was the original speaker of the utterance. Similarly,echo can be caused by hybrid 3, reflecting signals transmitted bytermination 2 back to the speaker using that termination. Echo can alsobe caused by acoustical feedback at the remote end, between the user'searpiece and mouthpiece.

The delay between the outgoing and incoming signal as perceived by theuser of a termination 1 is determined largely by the distance betweenthe termination 1 and the hybrid 4 or other element which is causing theecho. Similarly, the delay between the outgoing and incoming signal asperceived by the user of a termination 2 is determined largely by thedistance between the termination 2 and the hybrid 3 which is causing theecho.

The device 6 is connected to the network by tap connections X and Yconnected to paths 5, 15 respectively and is used to detect the presenceof echo in the system by monitoring both lines for signals, andcross-correlating these signals to identify characteristic signalshaving passed connection X and subsequently passing connection Y, orvice versa. Connections X and Y are simple low-impedance T-connectionsallowing the signals transmitted over the paths 5, 15 to be monitored bydevice 6. By measuring the delay between these events the distance ofthe source of echo can be derived: for example an echo generated byhybrid 4 produces a shorter echo delay than one from termination 2.Moreover, the path 5 or 15 on which the original signal appearedidentifies the direction in which the echo is coming from, therebyestablishing whether the source of echo is between device 6 and thefirst termination 1 or between the device 6 and the second termination2.

In a real network there would be several elements such as hybrids 3, 4on either side of the device 6, any of which might be the source ofecho.

The echo detection device 6 uses a cross-correlation technique tocompare speech on the reflected and transmitted paths. Cross-correlationis a method of statistical comparison of two signals generally used, insignal processing, to calculate the delay between the input waveform andthe output waveform of a system.

In the present case the system in question is the echo path of thetelephony circuit, i.e. from connection X to connection Y via hybrid 4,or from connection Y to connection X via hybrid 3.

The transmitted signal is compared with the reflected signal (normalisedin amplitude to correspond to that of the transmitted signal) and across-correlation coefficient is calculated. The cross-correlationcoefficient has a value from -1 to 1 and it describes how similar thetwo signals are. A value of 1 signifies a complete cross-correlation andresults when the two waveforms are identical. A value of -1 signifies acomplete negative match i.e. the signals are identical but for a 180°phase inversion. The human ear is not sensitive to phase, so for thepresent purpose a negative correlation is as important as a positiveone, as the human ear will detect either as an echo. Consequently, theabsolute magnitude of the correlation is used. The transmitted signal isthen delayed by one unit of time and the cross-correlation coefficientis recalculated. A match between two signals (i.e. the magnitude of thecross-correlation coefficient being close to unity) will occur when thedelayed transmit signal equals the reflected signal.

The echo detector pre-processes the speech signal before performingcross-correlation. This significantly improves the accuracy andreliability of the device by choosing segments that contain speech tocross-correlate with. In particular, because only selected segments areanalysed, they can be analysed in more detail. For example the elementaldelay imposed on the transmitted signal can proceed in smallerincrements, improving the accuracy of the delay measurement.

To improve the accuracy and reliability of the system pre-processing ofthe signals is performed to identify speech segments that are suitablefor cross-correlation. This pre-processing also identifies the directionof the talker's speech i.e. near-to-far or far-to-near. As speech is anessentially unidirectional means of communication (one person talks andthe other listens) the monitor 6 measures both echo paths (`X to Y` viahybrid 4 and `Y to X` via hybrid 3). To enable (almost) real-timemeasurements, parallel processing is used to divide the echo path intosegments.

From the monitoring point X the original signal is passed to a databuffer 8 which stores the incoming signals for the length of time forwhich measurements may be made with them. The data entering the bufferis monitored by a speech pre-processing unit 9 which identifies segmentssuitable for measurement and indicates to a measurement unit 10 whichsuch segments are present in the buffer 8. A second data buffer 18 andspeech pre-processing unit 19 monitor the signals passing throughmonitoring point Y.

The outputs of speech pre-processing units 9, 19 are compared in adirection identification unit 11. This unit compares certaincharacteristics of the signal such as signal power and length of speechsegment to determine which of the channels is carrying the originalsignal.

The measurement unit 10 uses the output of the direction indicating unit11 and the speech pre-processing units 9, 19 to select data from buffers8, 18 on which to carry out cross-correlation measurements. The resultsof these measurements are transmitted to a post-processing unit 12 whichmakes use of the cross-correlation results to take appropriate action.

The post-processing unit 12 may use the cross-correlation measurementsto generate an echo cancellation signal. This is done in canceller 7 or17 by extracting the input signal from buffer 8 or 18 respectively,attenuating and delaying it by amounts equivalent to the detected echosignal as measured in unit 12 and applying to the return path 15 or 5respectively a signal corresponding to the result of this process butout of phase with the detected signal by 180°. This applied signal iscombined in combiners 13, 14 respectively with the echo arriving on thereturn path 5 or 15 to produce a zero output. It should be noted thatthe echo-cancellation signal should be applied downstream of themeasurement points X, Y, to prevent the echo cancellation signal itselfforming part of the signal measured on the return path.

The post-processing unit 12 may generate information for networkmanagement purposes. The length of delay can be used, in conjunctionwith knowledge of the call routing, to identify the component causingthe echo, allowing remedial action to be taken. Alternatively, the callmay be diverted to another route, or abandoned.

The required time to resolve a single echo and delay measurement isdependant on the maximum delay to be resolved i.e. for 1 second delayafter a suitable speech segment is detected it takes 1 second toaccumulate the samples and a further period to do the processing. Byjudicious programming it is possible to reduce the processing timefurther but ultimately the processing time is still dependant on thenumber of samples required to be stored for the echo path.

To decrease the processing time a means of setting the measurement rangeis included. Using this technique the algorithm can run simultaneouslyover several digital signalling processors (DSPs) within the measurementunit 10, with each DSP searching a different measurement range. Forexample four DSPs may be used to process delay measurements of 1 second.Each DSP searches a 250 range for the echo path (0-250, 250-500,500-750, 750-1000), hence the limiting factor on the speech of themeasurement is now only 250 ms. If the algorithm is used for nationalnetworks where the upper limit delay is likely not to exceed 60 ms therange can be reduced accordingly.

This configuration is very suitable for parallel processing enabling thecorrelation to spread over several processors--this improves thespeed/efficiency of the algorithm.

A high level controller can determine which DSP returns the correctdelay value by examination of the cross-correlation coefficient.

This technique of dynamically allocating the algorithm across the DSPresources increases the number of successful measurements in a giventime period.

The buffers 8, 18 are used to store uncompressed samples from the 2Mbit/s streams in the paths 5, 15. The buffers use a two pointer FIFO(first in first out) buffer, which has two flags FULL and EMPTY.

A conversation is constructed from speech spurts and pauses. Speechspurts give the best cross-correlation as the attenuation due to theecho path will diminish low energy segments, such as unvoiced and noisesignals, the most. It is therefore important that the pre-processingselects segments that are likely to give a good cross-correlation.Pre-processing units 9, 19 select speech segments of the signals forcross-correlation.

A minimum segment length (40 ms) is required to give a reliable andaccurate cross-correlation. The reliability improves further if a longersegment is used although the improvement is negligible above 80 ms.However if a segment is a fixed length i.e. 80 ms, it may contain only ashort speech spurt at the beginning with the remainder of the segmentbeing noise. If this occurs the segment is less likely tocross-correlate. A variable segment length ensures that the segmentcontains mainly speech, not noise. The pre-processing selects segmentsof speech between 40 and 80 ms in length.

As conversations are essentially uni-directional--people take turns tospeak to each other--a direction indication unit 11 can be used todetect which party is talking. The echo path delay and loss is thencalculated for that direction i.e. if speech is detected at point `X`the echo path `X-4-Y` is calculated; conversely if speech is detected atpoint `Y` the echo path `Y-3-X` is calculated. If speech is only presentin one direction then it is not possible to resolve the echo path in theopposite direction.

The direction is found by comparing the length of the speech segments onthe two channels. The channel with the longest segment of speech istaken as the channel with the incident speech.

A standard cross-correlation algorithm is used to calculate the delay.

If the Speech Echo Path Delay (SEPD) is resolved, the incident signal isgiven a delay, equal to the SEPD, and the echo signal loss is calculatedfrom the difference between the root mean square (rms) of the incidentsignal and the rms of the reflected signal.

As mentioned above, speech needs to be present on a channel before ameasurement can be resolved. The minimum measurement time is 15 seconds.This will increase the probability that a suitable segment of speechwill be present on the channel. Within the 15 seconds it is likely thatseveral measurements will be made--some means is required to choose themeasurements that are correct. The method relies on two processes.Firstly cross-correlation produces a correlation coefficient value, orconfidence factor. If the signals match exactly after the signals havebeen normalised and delayed suitably, an exact match produces acorrelation coefficient of 1. Due to the impairments of the echo path itis likely that the correlation will be less than 1. Tests have shownthat provided the correlation value is greater than 0.5 then the delayhas been calculated correctly. Secondly if several results are producedit is reasonable to assume that each measurement is within an allowableaccuracy of each other. A rolling average is used so that a value isincluded in the average if at least two results are within the allowedaccuracy of each other. It is likely that any wrong cross-correlationwill produce random delay estimates and hence will not be included inthe average result.

In the embodiment described above cross-correlation is performed in thetime domain. Alternatively, it could be performed in the frequencydomain using fast Fourier transforms (FFT). This requires more memorybut is more efficient.

One simple method for calculating cross-correlation is to only use thesign bit of the signal. If the samples from the original and reflectedsignals are of the same sign a counter is incremented, if they are ofopposite sign the counter is decremented. For a good match a large totalwill be found, its magnitude being related to the length of the sampleand its sign dependent on whether the echo is in-phase or antiphase. Theoutput can be normalised using the length of the sample, giving valuesin the range -1 to +1. This method is not as accurate as other means ofcalculating the correlation coefficient but is reasonably accurate forlow level of loss. It has the advantage of not being as computationallyintensive and hence very quick. Such an arrangement is suitable forlower cost DSPs which have limited processing power and are designed tooperate on circuits that will have a lower echo loss value.

The method is not limited to using speech as the circuit stimulus (ithas however, been optimised for speech). Circuits that already have echocancellers present will not, under normal operation, have an echopresent. Although there is no echo present, round-trip delay is a usefulmeasurement to obtain. For these circuits a continuity signal, generatedby the signalling system can be used to perform the cross-correlation. Acontinuity signal is a tone, transmitted on the speech path from theoutgoing switch to the incoming switch which loops the signal back. Thismethod gives a measure of the delay between international switches.Continuity check tones are generated by the InternationalTelecommunications Union (ITU-T) signalling system number 7 prior to aringing tone.

The method of the invention can be applied to other applications notdirectly related to voice telephony, and in this specification the term`telecommunications link` is used in the broad sense to cover any linkcarrying signals from one point to another, whether as part of aswitched system or a dedicated link.

The interference detection system of the invention can be used forproviding the echo delay input for an echo loss calculator as will nowbe described.

In FIG. 3 a delayed incident signal 28 and reflected signal 31 are inputto a digital analogue filter 29. The output 32 of the DAF 29 is comparedwith a reflected signal 31 in a comparator 35 to generate an errorsignal 33 which is input to the DAF 29.

Using the unmodified incident speech 23 (delayed by the bulk delay 22)and reflected speech 30 as the inputs, the DAF 29 would converge toproduce the impulse response of the echo path. The impulse response ofthe echo path is effectively a model of the echo path, however the modelproduced will not be exact as it is dependant on the characteristics ofthe speech. A DAF will converge to its optimum state if a white noisesignal is used as its input. Therefore to improve the accuracy and thespeed of convergence a linear prediction unit 24 is used to perform aform of pre-emphasis to modify the delayed incident signal 23 andreflected signal 30 to the DAF, to "whiten" the signals. The delayedincident signal 13 is modified in a filter 26 to generate a modifieddelayed incident signal 28. Similarly, the reflected signal 30 ismodified in a filter 27 or generate a modified reflected signal 31. Themodified signals 28, 31 are used as inputs to the DAF 29.

Speech signals consist of voiced and unvoiced segments. The voicedsegments are high in energy and the samples are auto-correlated incontrast to the lower energy noise-like samples in the unvoicedsegments. These characteristics result in a poor convergence rate of theLMS (least mean squares) algorithm used by the DAF. As the unvoicedsegments are low in energy they tend to be corrupted by echo path noise,so the properties of the higher energy voiced segments have beenexploited to improve the performance of the LMS algorithm. In order todo this the delayed incident signal is supplied to an LPC (linearpredictive coding) analysis unit 24 which derives the coefficients of afilter H(z) having a frequency response similar to the frequencyspectrum of the incident signal. Such analysis is well-known in the art.Essentially it generates a series of coefficients which, when applied toa white noise signal, reproduce the voiced sound that was modelled. Inthis way it simulates the effect of the vocal tract on the essentiallywhite noise input to it by the speaker's lungs and windpipe. By applyingthe inverse function 1 /H(z) of this in filters 26, 27 a pseudo-whitenoise signal corresponding in energy to the original speech can begenerated.

The linear prediction unit 24 receives an input from the delayedincident speech signal 23. The sequence H(z) generated by the units 24is transmitted as an output 25 to filters 26, 27 which apply the inverseof the sequence H(z) to the delayed incident signal 23, and thereflected signal 30, to generate modified outputs 28, 31 respectively.

The delay imposed by buffer 22 is determined by the correlationtechnique described above and this delay is applied to the signal 21 bymeans of the variable delay buffer 22, such that the DAF 29 is centredon the delay echo path. The DAF 29 will then converge on the echo path.

If the delay period is predetermined in this manner, the DAF 29 can becentred on the echo path, hence requiring the filter to have a muchshorter length than if the delay imposed by buffer 22 were only anestimate.

It is desirable to test for echo on both channels of a two-waytelecommunications link, as echo may appear on either, or both,channels. It is therefore necessary to identify on which channel theincident signal is to be found, so that the correct signal is delayed.

Instead of operating using a delay period variable between zero and apredetermined maximum, the period can instead be made variable betweennegative and positive values of the maximum.

However, since this would require both positive and negative values ofdelay to be tested for, it would halve the number of delay periods ofdifferent magnitude which can be tested for. Instead, in a preferredarrangement the channel currently carrying the incident signal isidentified in a pre-characterising stage. In most cases a two-way voicelink is used by the talkers in turn. It is therefore possible toidentify which of the two channels is currently in use and monitor onlythe return channel for echoes. This can be done by identifying on whichof the two channels the strongest signals are occurring. This channel isidentified as the `incident` channel and the other one is therefore the`reflected` channel.

In the embodiment of FIG. 3 the speech classification is carried out bythe voice activity detector 11. The detector 11 identifies on which ofthe two channels X, Y the strongest signals are to be found, andcontrols switches 36, 37. Switch 36 is arranged to provide eitherchannel X or channel Y to the input 21, under the control of thedetector 11. Similarly switch 37 is arranged to provide either channel Xor channel Y to the input 30, also under the control of the detector 11.Detector 11 provides outputs such that when switch 36 is set to channelX, switch 37 is set to channel Y, and vice versa.

What is claimed is:
 1. An interference detection system for atelecommunications link having separate first and second channels, thesystem comprising:first monitoring means for monitoring signalstravelling over the first channel, second monitoring means formonitoring signals travelling over the second channel; comparison meansfor comparing the signals monitored by the first and second monitoringmeans for one or more delay periods to identify the presence ofinterference between the channels, the comparison means being arrangedto identify cross-correlations between the signals monitored by thefirst and second monitoring means, the first monitoring means includingmeans for detecting and selecting signal segments on the first channelhaving predetermined characteristics; and the comparison means beingarranged to identify cross-correlations between such characteristicsignal segments and the signals monitored by the second monitoringmeans, the selected signal segments having lengths corresponding to theduration of the predetermined characteristic.
 2. A system as in claim 1wherein:the selection means is arranged to select parts of the signalhaving the detected predetermined characteristics having a durationgreater than a predetermined minimum.
 3. A system as in claim 1, whereinthe comparison means comprises:a plurality of cross-correlation means,each cross-correlation means performing a cross-correlation for adifferent delay period, and delay measurement means for determining,from the outputs of the cross-correlation means, the magnitude of thedelay in the interference signal.
 4. A system as in claim 3,including:means for determining a rolling average from a predeterminednumber of measurements from the delay measurement means differing fromeach other by values less than a predetermined value.
 5. A system as inclaim 1, including a speech direction determination meanscomprising:means for determining on which channel the longest segmentsof signal having the detected characteristics occur.
 6. A system as inclaim 1, in association with an echo loss calculation device, andfurther including:means for generating an echo delay signal in responseto the identification of a cross-correlation for a given delay period,and means for transmitting the echo delay signal to the echo losscalculation device.
 7. A system as in claim 1, including:means forintroducing a cancellation signal into the second channel.
 8. A networkmanagement system including:at least one means for introducing acancellation signal into a channel on which interference is detected, aplurality of interference detection systems according to claim 1 eachassociated with a respective pair of channels, and means for selectingthe channel with which the cancellation means is associated in responseto interference detected on the channel by the respective detectionmeans.
 9. A network management system including an interferencedetection system as in claim 1, comprising:means for identifying, fromthe delay measured by the system, the network elements responsible forthe interference.
 10. A communications system having a plurality ofcommunication channels, and an interference detection system accordingto claim 1,the first and second monitoring means of the interferencedetection system being arranged to monitor one or more pairs of thecommunication channels.
 11. A communications system as in claim 10,wherein the pair or pairs of channels each comprise a two-waycommunications link, the system being arranged to detect echo.
 12. Amethod of detecting interference between channels on atelecommunications link having first and second channels, the methodcomprising the steps of:monitoring signals travelling over a firstchannel, monitoring signals travelling over a second channel, comparingthe signals for one or more delay periods to identify the presence ofinterference between the channels, identifying cross-correlationsbetween the signals carried by the first and second channels, detectingsignals having predetermined characteristics on the first channel,selecting segments of signals having said characteristics, andidentifying cross-correlations between such characteristic signalsegments and the signals carried by the second channel, the selectedsignal segments having lengths corresponding to the duration of thepredetermined characteristic.
 13. A method as in claim 12, wherein thesegments selected are of duration greater than a predetermined minimum.14. A method as in claim 12, wherein the cross-correlations areperformed for a plurality of delay periods to determine the magnitude ofthe delay.
 15. A method as in claim 14, wherein:a rolling average of thedetermined delay is recorded, the average being calculated from apredetermined number of delay measurements differing from each other byless than a predetermined value.
 16. A method as in claim 12,wherein:the channel to be monitored for the initial speech signal isidentified by monitoring both channels for signals having thepredetermined characteristics and determining on which channel thelongest segments having the predetermined characteristics occur.
 17. Amethod of measuring echo path loss, in which echo delay is determined asin the method of claim
 12. 18. A method of interference-cancellationcomprising detecting interference by the method of claim 12, furthercomprising:adding to the second channel a signal complementary to thesignal detected on the first channel and having the same delay andattenuation as the interference signal detected.
 19. A method ofinterference cancellation in a telecommunications system comprising aplurality of channel pairs, said method comprising:monitoring eachchannel pair for interference by the method of claim 12, selecting atleast one channel pair having the strongest interference signals, andapplying to the second channel of the at least one pair a signalcomplementary to the signal detected on the first channel and having thesame delay and attenuation as the interference signal detected.
 20. Amethod of monitoring a telecommunications network comprising:detectingthe presence of interference by a method according to claim 12, anddetermining, from the delay so measured, the location of the element ofthe network responsible for causing the interference.
 21. A method as inclaim 12 in which:the two channels form a two-way communications link,the method being such that the interference detected on the secondchannel is an echo of the signal on the first channel.